Thus far we have shown that in Hepatoma Tissue Culture Cells (HTC Cells) all of the proteins of the plasma membrane turn over as a unit, probably by interiorization of a membrane "packet" followed by fusion with a lysosome. In HTC cells and probably in all animal cells, this packet of degraded membrane is replaced from a pool of preexisting membrane. In HTC cells, we have identified this pool of already formed plasma membrane in an interior compartment of the cell, showed that it "chases" to the plasma membrane, measured the kinetics of its synthesis and appearance at the cell surface and measured its size. There is at least as much plasma membrane inside the cell as on the plasma membrane. We have purified to homogeneity from the plasma membrane of these tissue culture cells four integral membrane polypeptides, three of which are glycoproteins and one of which is a non-glycoprotein of 55,000 M.W. BIBLIOGRAPHIC REFERENCES: Tweto, J., Friedman, E., and Doyle, D. Proteins of the hepatoma tissue culture cell plasma membrane. J. Supramolecular Structure 4, 141-159, 1976. Tweto, J. and Doyle, D. Turnover of the plasma membrane proteins of hepatoma tissue culture cells. J. Biol. Chem. 251, 872-882, 1976.